How did the concentration of reactants affect chemical reactions in Claude-Louis Berthollet’s theory?

In his work Essai de statique chimique (1803), Claude-Louis Berthollet challenged the prevailing belief that chemical affinities had absolute values. Instead, he proposed that these affinities are modified by the physical conditions of a reaction, with the concentration of reagents serving as a primary factor in determining the final outcome and direction of a chemical process.1

Berthollet’s insight was prompted by observing natural chemical formations that contradicted laboratory results, leading him to conclude that the relative quantities of substances present could override traditional elective affinities. This assertion was a significant departure from contemporary chemical theory and served as a foundational step toward the later formulation of the law of mass action. By emphasizing that reactions seek a static equilibrium influenced by factors like concentration, temperature, and solubility, Berthollet helped shift the study of chemical reactions toward a more systematic approach to physical chemistry.1